NMR tube specifications

NMR sample tubes come in a variety of grades. Good quality tubes will not only give you better quality spectra, but are less likely to damage the instrument. In this post the specifications used to distinguish a good NMR tube from a bad one are explained and how these parameters affect your spectra are discussed.

NMR tube manufacturers produce tubes in disposable, economy and performance grades. The highest grade tubes are the straightest and have the most uniform wall thickness. The specifications used by manufacturers to assess tube quality are outer tube diameter, inner tube diameter, concentricity, and camber. These specifications are usually given in fractions of a millimeter or in microns.

Outer tube diameter is the distance across the NMR tube from the outer surface on one side to the outer surface on the other side. Inner tube diameter is measured in the same way but between the inner surfaces. These measurements are made at many points along the length of the tube and reported as a mean and standard deviation. If the outer tube diameter is too large, then the tube may contact the inner surface of the probe and perhaps damage it. If the outer diameter is too small, then the tube may slip through the spinner and again come into contact with the probe. In modern probes the space between the sample tube and the probe is around 0.3 mm, so there is not a lot of room to accommodate misshapen tubes.

Concentricity measures how well the cylinders defined by the inner and outer walls of the tube overlap and is reported as a deviation. A perfect tube will have zero concentricity. Variations in concentricity could lead to some areas of the sample being outside the cylindrical space where the magnetic field is uniform, which will lead to peak broadening.

Camber is a measure of the straightness of a tube and like concentricity is reported as a deviation. Tubes with high camber will not spin well and may come into contact with the probe coils, perhaps damaging them. They are also likely to place the sample outside the volume of the uniform magnetic field.

The table below shows the specifications of some different NMR tubes to give an idea of the range of values. Not all specifications are reported by all suppliers, so some cells are empty

Manufacturer	ID	Type	Outer diameter (mm)	Inner diameter (mm)	Concentricity (mm)	Camber (mm)	Recommended field (MHz)
Wilmad	WG-1241-7-5	5mm Economy	4.946 ± 0.019	4.516 ± 0.019	0.0038	0.0038	600
Wilmad	535-PP-7-5	5mm Precision	4.9635 ± 0.0065	4.2065 ± 0.0065	0.0013	0.0006	600
Norell	502-7	5mm Economy	4.97 ± 0.05	4.20 ± 0.05	0.020	0.070
Norell	509-UP-7	5mm Precision	4.97 ± 0.006	4.20 ± 0.012	0.004	0.006	600
New Era	NE-UL5-7	5mm Precision	4.960 ± 0.006	4.560 ± 0.006	0.003	0.003	500-700
Bruker	Z112273	5mm SampleJet	5.00	4.62		0.06
Bruker	Z106462	1.7mm SampleJet	1.70	1.50		0.06

As expected the economy tubes have much greater variation in outer and inner diameter and have larger concentricity and camber. The Wilmad tubes seem to have the best specs, consistent with their reputation. Many of the specifications for the Bruker SampleJet tubes were not available, but the camber numbers suggest these tubes are comparable to the economy tubes of the other manufacturers.